Question

A boy runs for 10min at a uniform speed of 9kmph . At what speed should he run for the next 20min so that the average speed comes 12kmph.

Answer 1

Let D1 be the distance travelled by the boy at the first 10 minute .D2 be the distance travelled by th boy in the next 20 minute.

Average speed (12km/hr) = Total Distance / Total Time
= (D1 + D2 )/( T1 + T2)
Total Time = 10 min + 20 min
= 30 min
= 1/2 Hour
D1 = Speed * Time
= 9 * 1/6
(10 min = 1/6 hr)
= 3/2 km
= 1.5 km
D2 = ?
12 = 1.5 + D2 / 1/2
12 * 1/2 = 1.5 + D2
6 = 1.5 + D2
6 - 1.5 = D2
4.5 km = D2
Speed = Distance / Time
= 4.5 / 1/3
(20 min = 1/3 hr)
= 4.5 * 3 (Reciprocal)
= 13.5 km/hr